Pressure regulating valves are used in myriad industrial and residential applications for controlling the downstream pressure of a fluid. For example, in chemical processing plants or oil refineries, pressure regulating valves are used to manipulate a flowing fluid to compensate for increases or decreases in demand, or other load disturbances, and thus keep the fluid pressure regulated. Similarly, pressure regulating valves may be used in plumbing fixtures to maintain a pre-determined pressure of fluid that automatically adjusts to variations in demand, such as anti-scald valves in showers or faucets. By controlling downstream pressure, pressure regulating valves compensate for variations in downstream demand. For example, as downstream demand increases, pressure regulating valves open to allow more fluid to flow through the pressure regulating valve, thus maintaining a relatively constant downstream pressure. On the other hand, as downstream demand decreases, pressure regulating valves close to reduce the amount of fluid flowing through the pressure regulating valve, again maintaining a relatively constant downstream pressure.
Pressure regulating valves can be categorized as either balanced or unbalanced. Unbalanced valves typically have high pressure inlet fluid on one side of the valve plug and lower pressure outlet fluid on the other side of the valve plug. Unbalanced valves suffer from an undesirable effect known as decaying inlet characteristic. The decaying inlet characteristic is a phenomenon in which an unbalanced valve experiences an unintended increase in downstream pressure as the upstream pressure decreases. This effect is undesirable as most pressure regulating valves attempt to maintain a constant downstream pressure. Decaying inlet characteristic is caused by fluid forces on the high pressure side of the valve plug attempting to move the valve plug to a closed position. As a result, the valve must have some mechanism to oppose this fluid force on the valve plug. Because the mechanism that opposes the fluid force typically has a set point, the force generated by such a mechanism is constant while the fluid force on the inlet side of the valve plug may vary (e.g., due to a decreasing supply of inlet fluid, or due to pressure variations upstream of the valve). Decaying inlet characteristic is particularly important to applications having a limited compressed fluid source, such as gas cylinders, tube trailers, or hydrils, because in such applications, there is a fixed supply of inlet fluid and thus, the inlet fluid pressure decreases as the inlet fluid supply decreases.
Unbalanced valves also suffer from damage that occurs to the valve seat. In unbalanced valves with high inlet pressures, the fluid pressure acting on large valve orifices can crush the valve seat. As a result, unbalanced valves are not ideal for high pressure, large orifice applications.
One way to minimize the decaying inlet characteristic is to minimize the difference between inlet and outlet pressure. Thus, multiple stage unbalanced pressure regulators, where each stage only reduces the pressure by a small amount, are less subject to the decaying inlet pressure characteristic. Multi-stage unbalanced pressure regulators are typically well suited for relatively low flow applications.
To address the decaying inlet characteristic in higher flow applications, balanced pressure regulating valves were developed. In the balanced valve, a portion of the downstream pressure is vented to act on an upstream portion of the valve plug. Thus, the valve plug is “balanced,” having the same fluid pressure act on both upstream and downstream portions of the valve plug. In this way, the decaying inlet characteristic is eliminated (or greatly reduced) because there is no difference in the fluid forces acting on valve plug surfaces both upstream and downstream of the valve seat that would tend to force the valve plug towards the closed position. In other words, the valve plug itself generates very little, or no opening/closing forces due to fluid pressures.
Recently, some unbalanced pressure reducing valves have been developed that include a valve cartridge. The valve cartridge may be assembled as a sub-assembly. The valve cartridge may then be installed into an unbalanced valve body as a unit. Some unbalanced valve cartridges include a valve plug, a valve seat, a retaining body, and a filter on the high pressure side of the valve plug. Because fluid flowing through the unbalanced cartridge is filtered the life of the unbalanced valve cartridge may be extended and reliability and precision of the unbalanced pressure regulating valve may be improved. While such valve cartridges have improved assembly and durability, these valve cartridges have been limited to unbalanced pressure reducing valves due to manufacturing and cost limitations.